This invention relates generally to implantable medical devices and their methods of use for stabilizing skeletal bone, and relates more particularly to fasteners for attaching implantable medical devices and their use for stabilizing the vertebrae of a human spine.
With normal anatomy, the vertebrae of the spinal column are held together and to the skeleton by a complex arrangement of ligaments, tendons and muscles. Degenerative diseases, deformities, or trauma may cause abnormal conditions. These problems generally cause or allow displacement or rotation of a vertebra relative to the adjacent vertebrae, or produce painful motion. When spinal discs tear, rupture or bulge the intervertebral space between two adjacent vertebras can decrease or displace abnormally and cause discomfort to the patient. When surgery is needed, the discs are replaced with implants that will heal or “fuse” the spine together. This device, with its associated stabilization, maintains the vertebral position while healing takes place. The result is referred to as “spinal fusion”. The objective of spinal implants is to facilitate or maintain realignment and/or fixation of spinal elements. Clinical studies have demonstrated that surgeries using spinal implants are more effective at maintaining alignment and providing rigidity to the spine than surgeries in which implants are not used. Since the introduction of stabilizers as crude plates, rods, and wires, these devices have been developed into sophisticated appliances, which can be assembled and configured to rigidize spines of any size or condition. These stabilizers also provide mechanical fixation for restraint of an implanted graft material. With this fixation, displacement during healing is significantly reduced thereby reducing the failure rate. Failure of the stabilizer commonly results from screw breakage.
The majority of existing cervical stabilizers use plates that are bent in both the radial plain to conform to the vertebrae, and along the spinal axes to maintain lordosis. Bicortical screw purchase has been favored because of the increased strength of the construct and increased screw thread area within the bone. These screws are more technically challenging to place and add increased risk of morbidity from neural canal penetration and screw backout. The reduced bending reaction strength and decreased thread area of a unicortical screw purchase increases the probability of screw back out or loosening resulting in esophageal injury. Unicortical purchase results in a single point fixed end cantilever construct versus bicortical purchase which results in a dual point fixed end construct. Screw back out and loosening has led to the development of mechanisms for locking the screw head to the plate in unicortical screw plate designs. Such locking mechanisms not only prevent screw back out, they also reduce the tendency of the screw head to pivot within the plate. Locking the screw to a plate, rod or other insert results in a fixed point of attachment.
A second point of fixation results from the screw portion fixed within the bone. This produces a stress raiser of greatest stress, just below the screw attachment at the plate, rod or other implant. This is the area of most frequent screw breakage observed in clinical practice and biomechanical testing. This area of screw breakage is well known to those practiced in the art. In the lumbar spine, posterior screws break between the second and third thread below the plate or rod. In the cervical spine the anterior screws break at a similar location. In the present invention, particular embodiments are described below, These embodiments improve screw strength and lock the screw within the bone by extending a tapered unthreaded section of the screw shank into the bone.